Crate V5 mods... anyone?

[jim p]

question on choke input mod

Was asking if the choke was getting warm when amp is converted to a choke input filter. Also what is the amplifier noise like? The amp I have with stock Pi filter is so quiet you don't know it is on.
Regarding headroom the amp I modified is fine did change 12AX7 to an Electro-Harmonix from the stock Sovtek. You can see by looking at the EH that it is not just a relabeled Sovetek.
What speakers have you played the amp through?

 

[goonrick]

Was asking if the choke was getting warm when amp is converted to a choke input filter. Also what is the amplifier noise like? The amp I have with stock Pi filter is so quiet you don't know it is on.
Regarding headroom the amp I modified is fine did change 12AX7 to an Electro-Harmonix from the stock Sovtek. You can see by looking at the EH that it is not just a relabeled Sovetek.
What speakers have you played the amp through?

Haven't checked on the choke. The amp has a low level 60 Hz hum (no buzz though) that it had since the moment I first plugged it in. It really is so low by comparison of the guitar signal that I'm not worried about it. From looking at the schematic, it appears that they used AC voltage on the power tube filament, and I'm betting this is the source of the noise. No hum reducing resistors to ground or anything. I looked the PC board over and it is too much trouble to change, so I'm not going to worry about it.

I'll pull the amp chassis from the cabinet and operate it at full volume for a while and check the choke temperature and report back.

BTW, I've plugged it into a 1 X 12" cabinet loaded with a Jensen Mod 12-70 and it sounds fantastic. I swapped the internal crate crap 8-30 for a Laney HH Invader speaker and a huge improvement there as well. It gets a nice, boxy tone which I expect from the type of enclosure it is. I simply love it.

 

[goonrick]

OK. After about 10 straight minutes of high gain playing, the choke is cool to the touch. I started out with the multimeter and temp probe ready, but since it's not even warm to the touch I skipped actually taking a reading. I'd say the choke input mod is stable.

 

[jim p]

good to know about input choke mod

OK. After about 10 straight minutes of high gain playing, the choke is cool to the touch. I started out with the multimeter and temp probe ready, but since it's not even warm to the touch I skipped actually taking a reading. I'd say the choke input mod is stable.

When I ran a circuit simulation it gave me 5 watts on the choke but I have been fooled by circuit simulations before. The ripple was higher in the choke input simulation also so the amp noise maybe higher from that. The amp I changed to a 6V6GT has no noticeable noise the other one may have a bad filter cap so it is hard to tell. For head room you could try some feedback on the triode stages say 27 to 40 pf (300 volt) from plate to grid. Not that I am totally pushing op amps but the one stage was inverting so you could go less then unity (X 1) if that was in the amp. Also you could do a resistor capacitor feedback on the input triode and really drop gain but would need the op amp as an input buffer for that.

 

[jim p]

Speaker choice on V5

For the speaker on the two V5 amps I have modified I have used Weber speakers in both the price is great and I think Weber designs speakers with tubes in mind. There is a lot more interaction between amp and speaker with a tube amp especially a single ended output type like this one. If you think of the speaker as walking a dog this type of amp is like having a six year old walk an Irish setter the dog will call the shots in some cases. While a push pull tube amp will move up to being a 12 year old walking the dog and a solid state amp is giving the job to a linebacker. A tube amp has high output impedance so the parasitic components of the speaker inductance and capacitance (also the transformers inductance and capacitance) have a greater effect on the output sound. You can see this if you have an oscilloscope and signal generator looking at the output with a speaker for the load vs. a resistor. The Ted Weber web site has a Q and A section that has a lot of good information there are also sound clips. One more thing they have is an amp load that is a speaker motor the microMass this would be a better way of overdriving the amp vs. a resistor and it has a line out and tone control.
If you have yet to buy am amp MF has Weber speakers so you could get both at the same time and save shipping.

 

[rock_mumbles]

Crate v5 speaker

A friend has an all tube (modded) V5 with a Weber Sig 10 ceramic in it, it sounds a whole lot better than the stock speaker, but today just for grins I hooked up his custom made harp amp cabinet with a Weber sig 8S alnico, it sounded a lot less "boxy" than the crate v5 with the Sig 10 ceramic and had a fuller sound. I would guess that's due to the much better speaker cabinet and smoother sounding alnico speaker.

The Weber Signature speakers are great values for the money, the 10" signature ceramic speaker fits into the V5 cabinet without any modifications. I have an Eminence 10" alnico (an older 1028 maybe) in my V5 it sounds better than the stock speaker, but I haven't tried it with an all tube mod yet, so I'm looking forward to see how it is with a modded amp

 

[deafelectromark]

V5 and onward

Hi Guys,
I have just read all the comments on this thread, and it looks like many of you are finding that this amplifier is a sleeper and can sound good if a few things can be changed. I thought I would drop a line here and tell you my reaction to all the stuff that is suggested here.
First thing (almost always) the speaker must go. it's cheap and plastic and would make a great speaker for the beach or camping. I am sure the cone could withstand water, but the sound is very unmusical for serious playing.
Second thing is that adding more parts, or trying to make a 'booster box' in the amp with the 'provided' op-amp is more technical than most owners could do confidently. Changes should be in small steps so that you know what it is that you did that made (or lost) that sound.
I will summarize my mod again for those who did not catch it in an earlier listing. I am not an Electronics Designer of Analog Audio, but I have been blowing up stuff before I was ten and did earn a Electronics Technician Certificate to fill in some of the things that I didn't know (about theory, design, etc,) and have worked in TV repair, High-End Hi-Fi ($$$) and custom installations while building, testing and tweaking amps.
I tend to over mod things and it becomes less satisfying to keep on modding things to death. Trying to make a cheap little amp sound like a Marshall stack is not going to happen. But if you want to get much better sound for the fewest bucks, you need to get the 'sand' out of the circuit. I want to be connected to the speaker, not dealing with quantum leaps inside a plastic part.
When the op amp was removed, there was very little gain to play with. You can get output tube distortion if you play it hard, with hot pickups and perhaps your favorite stomp box. I feel that some of the best distortion comes from those little boxes on the floor. 2 tubes are not going to get you a lot of gain (unlike my Epiphone VJ that could through careful parts section) so why try and make things harder for the amp, your time and money, and your general satisfaction of PLAYING, not tweaking. I do both- and I do admit I play MUCH more than I tinker. My amp sounds very good, and there isn't anything in it to make it solid-state, but I do some tricks in front of it.
My pedal layout in guitar into a homemade tube distortion box with one 12AX7 in it, then a dbx 119 compressor/expander, and then to a Picoverb for lushness. That all goes in the to the amp with the controls the way they are printed (stock look) and no other holes to drill, etc.
Get a copy of the schematic and layout and just try this if you really want all-tube sound. If you want metal, or loud headroom, you will not get it from this amp. But it is a great case, chassis and transformers just begging to make music.
Begin by cutting the trace after the input jack leaving R10,R11 for input loading. This will be just before pin 3 of op amp. Run a short length of coax from there to pin 2 of the 12AX7, and ground this wire at only one end (to prevent ground loops).
You will be cutting the volume pot out of the circuit and reroute wires as follows-keep the tone control circuit for now).
Connect the middle wiper to pin 2 of 12AX7. Cut out the R7, 220K resistor and the pot will take it's place for the load and volume control BETWEEN the two 12AX7 halves (this is so your guitar and/or you stompbox can play clean into the preamp tube or to push it more into output stage distortion)
Connect the top of the control to the junction of R15, R29,and C28: and the bottom to ground. Cut out the 100K (R15) (or R27, 10K) Resistor(s) so that the second half of the preamp tube is driving the same load. So you are basically removing R15, R27 and R7. You can increase the gain by floating a highish value resistor between the grounded end of the volume control (or a 500K or more pot) which gives you more drive voltage out of the second stage of the preamp, but I left that the way it is because it does give me the balanced sound (clean and dirty) that I want.
No parts, but a few pieces of wire, a speaker, and your time.
If you don't feel comfortable doing this, I still offer to do it anyway you want. (shipping is least expensive if you just send me your board and i will do it all for you and ship it back to you ($5.00 shipping) you put the board back in. Alternately, you can take the chassis out of the box and ship that to me, so you don't have to do anything other than unscrewing it back together, and the shipping wont me too bad.
Mark (manoteal at cox dot net)

 

[jim p]

Weber alnico will fit V5 also

From my post stated the Weber alnico fits the V5 just need to remove the magnet cover if the speaker has one. When you say boxy how do you mean. An 8 inch speaker will have higher frequency response then a 10 so is it the high end? While the 10 will have more bass. The frequency response range relative to cone diameter is suppose to be speed of sound(13584 inches/sec in air) divided by diameter for high end and low frequency is that divided by 10. So the 10 inch is 13584 in/sec/10in for 1358 Hz to 1353/10 for 135 Hz low end above this and below are harmonics and the sound being distorted somewhat. From the numbers it looks that a guitar amp should have a 10 to 12 inch speaker for bass and a 6 inch speaker for treble but from the Weber Q and A it looks that they have tried to work around this in there speaker design.

 

[jim p]

Gain in the amplifier

I think there is some misunderstanding of the gain in this amplifier doing basic calculations total impedance at the plate of the input 12AX7 times transconductance it has a gain of approx of 30 to 40 (don’t want to figure exact with tone stack it is frequency dependent to the extreme). The second triode not loaded down by a tone stack has a gain of approx 60. Looking at the data sheet for an EL84 from Mullard circa 1961 (find at tubedata.org) you only need 4.4 volts rms for five watts out. So dividing 4.4 volts rms by 60 you need just 73mV going into the second tube for full volume. You will never overdrive the second tube at that rate. On the guitar end a note is suppose to produce 100mV rms while a chord is good for 1V rms. So if you plugged straight into the grid of the EL84 you could get ¼ output from a strummed chord. If you want to go basic you just need one triode and one pentode to go from guitar to transformer and speaker with gain to spare. Check out old amplifier schematics at schematic heaven they were one input pentode and one output pentode. I think using both triodes is done because they are there you get two may as well use them.
If you want a lot of gain use one triode as a current source load for the other that will get you close to the triodes amplification factor of 100.
So bottom line you always over drive the first tube when using overdrive along with or without the EL84 output tube. The tone stack is a gain killer for the first tube and the amp has more gain then is needed.

 

[rock_mumbles]

When you say boxy how do you mean. An 8 inch speaker will have higher frequency response then a 10 so is it the high end? While the 10 will have more bass.

Hi Jim, first thanks for all the modding info ...

I think that the above statement was addressed to me.

With the V5 I played with yesterday, the 10" ceramic sig in the V5 cabinet sounded more "megaphone" like (boxy - unnatural midrange from too small of a cabinet) than the alnico 8" sig that is in a larger semi-closed back cabinet. The 8" speaker has more bottom end in the "custom" harp amp cabinet than the larger 10" speaker in the stock V5 open back cabinet. This shows the effect of a good cabinet

 

[goonrick]

I think there is some misunderstanding of the gain in this amplifier doing basic calculations total impedance at the plate of the input 12AX7 times transconductance it has a gain of approx of 30 to 40 (don’t want to figure exact with tone stack it is frequency dependent to the extreme). The second triode not loaded down by a tone stack has a gain of approx 60. Looking at the data sheet for an EL84 from Mullard circa 1961 (find at tubedata.org) you only need 4.4 volts rms for five watts out. So dividing 4.4 volts rms by 60 you need just 73mV going into the second tube for full volume. You will never overdrive the second tube at that rate. On the guitar end a note is suppose to produce 100mV rms while a chord is good for 1V rms. So if you plugged straight into the grid of the EL84 you could get ¼ output from a strummed chord. If you want to go basic you just need one triode and one pentode to go from guitar to transformer and speaker with gain to spare. Check out old amplifier schematics at schematic heaven they were one input pentode and one output pentode. I think using both triodes is done because they are there you get two may as well use them.
If you want a lot of gain use one triode as a current source load for the other that will get you close to the triodes amplification factor of 100.
So bottom line you always over drive the first tube when using overdrive along with or without the EL84 output tube. The tone stack is a gain killer for the first tube and the amp has more gain then is needed.

Would removing the cathode capacitor on the first triode stage have enough of an effect to reduce how much that stage is being driven? Also, could one place a smaller value capacitor in its place to add high end emphasis so the tone stack can still cut and boost? Which value would you recommend to add a little sparkle?

Thanks for all the valuable info. I'm having a blast hacking into this little amp.

 

[jim p]

adding more treble

Would removing the cathode capacitor on the first triode stage have enough of an effect to reduce how much that stage is being driven? Also, could one place a smaller value capacitor in its place to add high end emphasis so the tone stack can still cut and boost? Which value would you recommend to add a little sparkle?

Thanks for all the valuable info. I'm having a blast hacking into this little amp.

I posted a schematic for an all tube tone stack what you want is a voltage divider where the bass has more resistance to deal with then the treble. So in that schematic you will see the 330k with 22k and 1nf across the low frequency sees 330k so 40% of the signal at the plate is across the volume pot. Then at about 500 Hz the cap starts to short out the resistor so all the signal at the plate is across the volume pot while at the same time the tube is losing gain because the load resistance is going down (chicken and egg thing). With any tube tone stack connected to the plate as you change the tone you also change the gain. Life is easer when the tone stack is connected to a cathode follower. Removing the cathode bypass resistor will drop the tube gain but not as much as doing the tone stack as I have in the schematic. The value for the bypass to start at 500 Hz is 200nf. (Equal to 1/ 2 pi (6.28) X 500 Hz X 1.5 k ohms = 212nf)

 

[jim p]

correction on cathode bypass

I posted a schematic for an all tube tone stack what you want is a voltage divider where the bass has more resistance to deal with then the treble. So in that schematic you will see the 330k with 22k and 1nf across the low frequency sees 330k so 40% of the signal at the plate is across the volume pot. Then at about 500 Hz the cap starts to short out the resistor so all the signal at the plate is across the volume pot while at the same time the tube is losing gain because the load resistance is going down (chicken and egg thing). With any tube tone stack connected to the plate as you change the tone you also change the gain. Life is easer when the tone stack is connected to a cathode follower. Removing the cathode bypass resistor will drop the tube gain but not as much as doing the tone stack as I have in the schematic. The value for the bypass to start at 500 Hz is 200nf. (Equal to 1/ 2 pi (6.28) X 500 Hz X 1.5 k ohms = 212nf)

Looked around the web and in a design book I have and using the bypass cap to change the low frequency gain it may give you a factor of two difference in gain. It all depends on the total plate impedance the higher the impedance the less the effect. What does you tone stack volume pot ect look like as far as values and layout.

 

[goonrick]

Looked around the web and in a design book I have and using the bypass cap to change the low frequency gain it may give you a factor of two difference in gain. It all depends on the total plate impedance the higher the impedance the less the effect. What does you tone stack volume pot ect look like as far as values and layout.

Ok, what I've done is bypass the op-amp stage and routed the signal directly to the first triode stage. In that stage, I kept the resistor values the same but removed the cathode capacitor and placed a .02uF cap for brighness. I left the tone stack the same except I put the 56p cap that was in C27 and placed it in C28. I removed R15 and R27 and placed the volume potentiometer in place of them.

Apart from the preamp, I modified the output section according to your notes and have gotten rid of the harsh clipping that was occurring and every pin on the el84 measures within specifications. It is now a choke input power section which runs cool and I pulled C14 and C15 and put a larger supply cap in C15's spot.

As of right now, the amp sounds really good with a 12AT7. A 12AX7 gooses the gain a bit but sounds a bit darker, probably due to increases in internal resistance. I'm very happy with both the gain available and tone from the AT, though, and will keep it in there.

I still get a 60Hz hum at idle, which was present since before I modified it to be a choke input amp. I think I'll rectify and filter the filament voltage on the el84 simply by cutting and splicing into the lead wires coming off the power transformer. Shouldn't be too hard and the result will be an amp with DC filament voltages on both tubes.

 

[deafelectromark]

Boxes and more boxes

[/QUOTE]From the numbers it looks that a guitar amp should have a 10 to 12 inch speaker for bass and a 6 inch speaker for treble but from the Weber Q and A it looks that they have tried to work around this in there speaker design.[/QUOTE]

As many know, a 4 inch speaker can sound good and a 15 can sound bad. There are so many variables, you cannot work this out 'by the numbers'. The box the speaker is in, whether it is stuffed or not, ported, open or sealed, the amp, the effects used, the pickups used, where they are located and the controls for those pickups, the strings used, the guitar body characteristics, but MOSTLY- the players style and attack. How many of you tend to be extra light on the lower strings with the neck pickup and more intensely play on the bridge pickup? I am the sole determiner of what I want my sound to be, and adjust to the variables listed above. It is instinctive or at least learned through many years of playing. We play the way we want, and can get there many ways with many speakers/amp/pickup, etc. combination.
Science is fun, but it can only get you so far.
Deafelectro:rockon:

 

[jim p]

Cathode bypass value?

Ok, what I've done is bypass the op-amp stage and routed the signal directly to the first triode stage. In that stage, I kept the resistor values the same but removed the cathode capacitor and placed a .02uF cap for brighness. I left the tone stack the same except I put the 56p cap that was in C27 and placed it in C28. I removed R15 and R27 and placed the volume potentiometer in place of them.

You wrote that you have a 0.02 uf for a cathode bypass is this C1? 20nf (0.02uf) won't have an effect until approx 5kHz maybe you want 200nf (0.2uf) there. On your connection to pin 7 of the triode did you remove R2 the 100k to ground? Having R1 R10 and R11 as stock is ok but only want 100k in there with the op amp driving things. Me I don't like C4 the whole treble up then down thing as you go clockwise is odd to me. Both C5 and C28 should have little to no effect overall. Maybe I will simulate the first stage tone stack combination to get an idea of what would work best for treble boost regarding the cathode bypassing.

Attached a schematic with cathode bypassing for treble boost. Had to split the cathode resistor to keep gain difference down. Changed value of C5 it is off by factor of 10 looks like should be 220pf can also be omitted. Left out C4 and C28 C28 has little effect find C4 to be odd.

 

[goonrick]

You wrote that you have a 0.02 uf for a cathode bypass is this C1? 20nf (0.02uf) won't have an effect until approx 5kHz maybe you want 200nf (0.2uf) there. On your connection to pin 7 of the triode did you remove R2 the 100k to ground? Having R1 R10 and R11 as stock is ok but only want 100k in there with the op amp driving things. Me I don't like C4 the whole treble up then down thing as you go clockwise is odd to me. Both C5 and C28 should have little to no effect overall. Maybe I will simulate the first stage tone stack combination to get an idea of what would work best for treble boost regarding the cathode bypassing...

I removed R10 and jumpered to the junction of R1 and R2. I also changed R2 to 1 megohm.

On the cathode bypass cap, I only wanted the boost to be on the high end, and settled on that value after trying 2.2uF and .1uF. It really seemed to give it just enough high end without getting shrill and it took the gain of that triode stage down so that I could get more out of the later stages without overdriving it too much. It took the amp from behaving like a little Marshall to more like a champ. I intended all along to use an overdrive, so I'm pretty happy with the results.

I didn't mind the stock tone stack so much as it is a bit weird, but has a few useful positions and seems to mesh with the new arrangement of the first triode stage.

Other than the low-level 60Hz hum, the amp is sounding great.

Does the 6v6 you retrofitted feature a spiral filament? That may be one of the reasons yours is so whisper quiet when running at idle. I'm going to rectify and filter the el84's filament voltage in an attempt to cut down on the hum.

 

[deafelectromark]

In the above schematic that JimP shows, there is only 1/3 of the available voltage going into the next stage. His 220K+220K+ another 220K with volume control in parallel with the last 220K will suck off 2/3 of the signal AT LEAST. The output stage of that section will work with as low as 100K, but 220K would be better. I put the volume control where the last of his 220K resistors is connected in parallel withe the volume pot and eliminate the 2 upper resistors and the grid leak with one volume control (I used the stock one in the amp to prove that it was fine for this. If you were to prefer more gain, then a 500K or higher pot will load the preceding plate less and you can get (theoretically) more gain, but with less grid leak, you might also get more noise and/or biasing issues). Since the leak resistor (the one that goes from the grid to ground) is 220K, putting a 250K pot would not alter stability and getting rid of the 440K (220K x2) losses on the way to the volume control and grid of next stage. That was the worst part of the circuit in stock form- only the original was wasting 90% of the signal before passing it along. If you notice the part numbers starting with R1, R2, etc that are nearest to the tube part of the circuit are lower than the ones around the opamp, they appear to be added later. The opamp provided so much gain (supposed to be a good thing, right?) that the designers had to change things around to suit it. At one point in time, they probably had it closer to "correct", but some designer messed it up with the opamp 'sand' in the system. If you look at highly regarded and good sounding amps, simpler IS better.
Deafguy

 

[jim p]

About the V5 son of VC508

If you call or e-mail crate for a schematic of the VC508 you will find that this amp is a stripped down version of it. The bad thing is they used the gain section (the op amp) for the volume control. Relative to how much signal you need going into the second triode it has a gain of approx 52 the maximum signal you could drive into the EL84 before grid current would occur is twice the cathode voltage (approx 15 volts) this is equal to 30 volts p-p. (By the data sheet 5 watts is with 12.3 volts p-p) so 0.58 volts p-p at the input to the second triode (will get you 30 V p-p). The lower the impedance of the tone stack the lower the gain of the first stage due to reflected impedance so lower values there will reduce the gain of the first stage. The capacitor values in the tone stack are biased on the impedance values in the stock circuit lower values should use larger value capacitors to compensate also the value of the tone pot may also need to be changed. The volume pot is a log pot so 12 o’clock is just 25k ohms (10%) out of 250k ohms with R7 in parallel that is 22.2k at the input to the second triode.
In the modifications that I have done the total gain of the op amp stages is -1.3 to correct for the pre emphasis circuit added at C25. Wanted to be certain that the first triode would clip well before the op amp. Also used the op amp as a high pass filter to limit frequencies below 120 Hz upper e string on a guitar is 190Hz helps cut down on that 60Hz hum.
This amp is not perfect if it were we not be buying it for around 90 bucks new (what did this cost to make?)
What is better then the VC805 is a 10 inch speaker stock it is junk but still the amp was only 90 bucks and you can choose one you like. The Pi filter on the plate supply, another thing price a choke for your amp it would cost you a minimum 16 bucks don’t have them on the valve jr’s I have.
+/- 15 volt power supplies which you can get +/- 18 volts from if you need more headroom. The VC508 used the 6.3 filament supply for just +/- 7 volts
A DC filament supply on the 12AX7.
So something you can mess with, make your own and costs less then you could buy the parts for yourself, cool.

PS: Regarding over all gain the first stage gain is approx 44 the second stage 52 the combined gain divided by the pot is 44 times 0.21 (pot in divider) times 52 equal to 480 the EL84 needs 4.4 rms for 5 watts so 4.4 divided by 480 equal to 9.1mV rms at input for full volume output if you remove R7 that goes to 828 for 5.0 mV rms. I think that is plenty of gain even if you are into tapping like Stanley Jordan.

 

[rock_mumbles]

C3 and placement of R16

Hello guys,

A couple of things, I've modded 3 Valve Juniors (all different) I like the sound of all three, the V5 has a lot more high end (too much to my ears) and also has a bit more gain than I'd like.

So, can anyone tell me what the purpose of C3 is? It's the 220pf cap across R16.

About the placement of R16. On a V5 R16 goes to ground after R31, what effect would there be if the 220K resistor was placed to ground between C7 and R31 instead of after R31?

Thanks!

 

[jim p]

Value of R16

Hello guys,

A couple of things, I've modded 3 Valve Juniors (all different) I like the sound of all three, the V5 has a lot more high end (too much to my ears) and also has a bit more gain than I'd like.

So, can anyone tell me what the purpose of C3 is? It's the 220pf cap across R16.

About the placement of R16. On a V5 R16 goes to ground after R31, what effect would there be if the 220K resistor was placed to ground between C7 and R31 instead of after R31?

Thanks!

C3 acrost R16 has no effect until approx 3.3 kHz it is out of the range of the normal frequencies from the guitar probably to roll things off when you overdrive the input. Moving R16 will have no real effect the pentode has little to no feedback capacitance (Miller effect) due to the screen grid between the plate and the cathode. A resistor in series with a triode is a different story there a resistor in series with the control grid will provide high frequency roll off due to plate to grid capacitance that is multiplied by the tube gain (Miller effect). R31 will limit the possible grid current and provide negative feedback if you overdrive the pentode (nasty thing to do to a tube the triode won’t like it either). What have you done with the op amp and tone stack? Without the op amp the tone control is just a treble cut there is no treble boost in the amp without the op amp or adding some. You could add a cap from the plate to the grid on the triodes this will lower the highs I would start at around 30 pF.
To drop the gain you could split the cathode resistors on the triodes I would have to work out the cathode impedance for the exact amount of change assume that it is 1.5k so if you remove bypass caps C1 or C8 you will half the gain of that stage ballpark current gain is 55 for each. So unbypassed that would drop to approx 28. If you use two resistors in series for the cathode resistor you can bypass one to adjust the gain say two 750 ohm resistors one bypassed would drop the gain by 0.67 X 55 = 37. (1500/1500 + 750 = 0.67) I was thinking someone might like to have this as a switch able function for the first triode. High gain when you want to over drive the tube low when you want more headroom

 

[jim p]

Source of hum

Other than the low-level 60Hz hum, the amp is sounding great.

Does the 6v6 you retrofitted feature a spiral filament? That may be one of the reasons yours is so whisper quiet when running at idle. I'm going to rectify and filter the el84's filament voltage in an attempt to cut down on the hum.

On the noise front is it for sure 60Hz as a sanity check and point of reference you can turn the amp volume down all the way plug in to the amp and put your finger on the tip of the jack then turn the volume up same frequency or higher from the amp? The filament supply is 60Hz while the plate supply is 120Hz so if it is power supply ripple on the plate it will be 120Hz. If you have added a standby switch another test would be to listen to the amplifier carefully right after you put it in standby. If the hum stops while the filter caps are still powering the amp it is the plate supply not the filament supply causing the noise the amp should still be active for a second or two after you flip the switch. On the 6V6GT filament winding I do not know but I think most tubes have opposing heater windings to prevent noise. The EL84 amp I moded doesn’t have any hum and the filter cap has self healed somewhat so that noise is down. The choke input may just be higher ripple then the pi filter supply. The Pi filter input cap would look like 28 ohms to ground to the 120 Hz and the choke would be 750 ohms in series followed by another cap that looks like 28 ohms. Plus the ripple into the choke would be small in amplititude due to the input cap.

 

[rock_mumbles]

Thanks for the info about C3 and R16 in post #111.

What have you done with the op amp and tone stack? Without the op amp the tone control is just a treble cut there is no treble boost in the amp without the op amp or adding some. You could add a cap from the plate to the grid on the triodes this will lower the highs I would start at around 30 pF.
To drop the gain you could split the cathode resistors on the triodes I would have to work out the cathode impedance for the exact amount of change assume that it is 1.5k so if you remove bypass caps C1 or C8 you will half the gain of that stage ballpark current gain is 55 for each. So unbypassed that would drop to approx 28. If you use two resistors in series for the cathode resistor you can bypass one to adjust the gain say two 750 ohm resistors one bypassed would drop the gain by 0.67 X 55 = 37. (1500/1500 + 750 = 0.67) I was thinking someone might like to have this as a switch able function for the first triode. High gain when you want to over drive the tube low when you want more headroom

I now have a blue eminence alnico 1028 speaker in my amp, and it's probably not really the amount of high end that bothers me it's the level of overdrive.

I have bypassed the op amp and have replaced C2 with a 0.022uf 715 Orange Drop which feeds into an 18 watt LiteIIb volume and tone control which then connects back to R7.

I just removed C8 on a friends V5 and wow did that clean it up, nice bright cleans at lower volumes that break up with higher volumes and harder attack, now it dimed is about the same gain as my amp on two or three. Since we both like more of a blues/classic rock sound, it's a lot better amp cleaned up.

The series cathode resistors sound really interesting. I could see that something between the bypassed gain level and the unbypassed gain level could be really useful. Would you "mess" with the gain on the first triode instead of the second? Why?? We were looking at a Princeton schematic so we removed the second bypass cap (I know that there's no cathode bypass cap because of the negative feedback in the princeton.) Would you still use the same bypass cap value, a 2.2uf like stock on the first triode?

Thanks again!

 

[amptweaker]

white noise

I would suspect that tantalum as your noise source.
I wouldn't worry too much about the high screen voltages as long as you use the stock Sovtek. These tubes look identical to the 6P14P and folks are using theme as 7189 subs. Besides, the 300 volts is a design center value for the 6BQ5.

On the amp I have I am reading a screen grid voltage of over 300 volts and all the data sheets rate the EL84 for a maximum voltage of 300 volts anyone have an idea on this. For now I have increased the screen grid resistor R16 to 10k dropping the voltage by 40 volts to 285 so 5k would put it right at 300 volts. This change also lowered the tube gain lowering the plate current also.
The other thing I have going is some white noise I replaced the ceramic cap C3 with a 500 pF mica paralleled C15 with 80uF, paralleled C9 with a 47uF tant and tried two other EL84 tubes but still have it. Anyone else have this experience?
The other amp I have that has a 6V6GT as the output tube is so quiet I think I am only hearing the dc filament voltage on the output tube so may try changing it to DC.

In looking over the schematic again I think the real problem with the screen grid voltage is it is connected to the wrong location. It should be connected to the junction of R26, R25 and C16 I have not made this change yet but will and post the results.

 

[amptweaker]

white noise

I would suspect that tantalum as your noise source.
I wouldn't worry too much about the high screen voltages as long as you use the stock Sovtek. These tubes look identical to the 6P14P and folks are using theme as 7189 subs. Besides, the 300 volts is a design center value for the 6BQ5.

On the amp I have I am reading a screen grid voltage of over 300 volts and all the data sheets rate the EL84 for a maximum voltage of 300 volts anyone have an idea on this. For now I have increased the screen grid resistor R16 to 10k dropping the voltage by 40 volts to 285 so 5k would put it right at 300 volts. This change also lowered the tube gain lowering the plate current also.
The other thing I have going is some white noise I replaced the ceramic cap C3 with a 500 pF mica paralleled C15 with 80uF, paralleled C9 with a 47uF tant and tried two other EL84 tubes but still have it. Anyone else have this experience?
The other amp I have that has a 6V6GT as the output tube is so quiet I think I am only hearing the dc filament voltage on the output tube so may try changing it to DC.

In looking over the schematic again I think the real problem with the screen grid voltage is it is connected to the wrong location. It should be connected to the junction of R26, R25 and C16 I have not made this change yet but will and post the results.

 

[jim p]

noise and screen grid voltage

I would suspect that tantalum as your noise source.
I wouldn't worry too much about the high screen voltages as long as you use the stock Sovtek. These tubes look identical to the 6P14P and folks are using theme as 7189 subs. Besides, the 300 volts is a design center value for the 6BQ5.

I am not looking for maximum gain from the EL84 so moving the screen grid to what looks to be it's proper location works for me. I have it connected to a triode/pentode switch anyway. I was using the tant to try to stop the noise by paralleling the cap already in circuit with no effect. Believe it to be the cap after the choke it has started to quiet so the dielectric is self healing. If I get a chance will swap the 47uF cap moving it to the input to the choke. But I have reverb drive and recovery and triode/pentode switch hardwired to the pcb so it makes that more of a job.

 

[amptweaker]

Speaker for V5

I'm doing a lot of testing on my V5 before I start to mod.

One of the first things I did was to measure and then calculate the impedence of the output transformer.
It's about 3K at 8 ohms.

I thought this was a bit low, and checked the data on the EL84.
A look at the Rodenhuis book confirmed it was (he uses 4.5 to 7 k).

Feeding the grid of the EL84 with the signal generator and using a dummy loads, i scoped it out. The cathode was 14.3volts at idle.

With the load at 8 ohms I got a 10vpp signal that had very noticible second harmonic distortion.
That is only 1.5 watts!
Increasing the input signal caused the bias to increase (rectification effect).
Nearly 17 volt on the cathode when the other side of the wave began to round at near 17.5vpp out. (4.78 watts into 8 ohm)

Testing at a 16 ohm load, the waveform was much more linear up to 22.5 vpp with less than 1.5 volt increase on the cathode. (4 watts at 16 ohm).
Increasing the input signal gave more symetric clipping (3rd harmonic).

I hope that I have shown that depending on the flavor of distortion you prefer, it is ok to use either an 8 or 16 ohm speaker.
As for raw wattage you will lose a bit with the 16 ohm speaker, but if the 16 ohm speaker is more efficient than the 8 ohm you will gain it back in perceived loudness.

I don't think the stock speaker was designed to be efficient much less handle 30 watts...
(I could smell the voice coil burning when i hooked it up to a 25 watt amp to break it in!)

 

[amptweaker]

Screen Voltage

I am not looking for maximum gain from the EL84 so moving the screen grid to what looks to be it's proper location works for me.

I noticed the unused tap at R25 and R26 too.

Never know what kind of whackyness you'll find in a Crate.

Took a look at the screen grid while driving it hard and it does turn cherry red...

 

[jim p]

cathode bypass ideas

Thanks for the info about C3 and R16 in post #111.



I have bypassed the op amp and have replaced C2 with a 0.022uf 715 Orange Drop which feeds into an 18 watt LiteIIb volume and tone control which then connects back to R7.

I just removed C8 on a friends V5 and wow did that clean it up, nice bright cleans at lower volumes that break up with higher volumes and harder attack, now it dimed is about the same gain as my amp on two or three. Since we both like more of a blues/classic rock sound, it's a lot better amp cleaned up.

The series cathode resistors sound really interesting. I could see that something between the bypassed gain level and the unbypassed gain level could be really useful. Would you "mess" with the gain on the first triode instead of the second? Why?? We were looking at a Princeton schematic so we removed the second bypass cap (I know that there's no cathode bypass cap because of the negative feedback in the princeton.) Would you still use the same bypass cap value, a 2.2uf like stock on the first triode?

Thanks again!

Replacing C2 with a lower value should have increased the highs out of the amp (C8 being a high pass component) but this is dependent on the tone stack you have. Can I find a schematic for your tone stack on the web?
Removing C8 is interesting that should increase the high frequency range of the tube (reduce gain equals reduced Miller capacitance) but you probably now increase the setting on the volume pot thus the input resistance and are back to where you started in that regard. But having no bypass will add degenerative feedback so maybe the tube is more stable.
On the split bypass if you use it on the input triode and know you do not want to overdrive the amp you could set it for unbypassed this would lower the gain so no overdriving the tube. Also lower gain is lower Miller capacitance so you won’t loose the highs when plugged straight into the guitar from that. The tube response may also be a bit more stable from the feedback. Some people are posting that they are changing to a 12AY7 or 12AT7 for lower gain this may work just as well for them.
Then if you want to overdrive the input triode you would bypass the cathode resistor for full gain on the input triode. So you just need to connect the ground lead of the bypass capacitor to ground through a switch.
Also some unbypassed resistance in series with the cathode might make things more stable from the feedback it would provide I have to search to see if that has been done.
On the bypassing of the second triode it would just be to reduce overall gain to remove it, except as before it might provide stability. One thing that may or may not be a problem is both triodes share the same decoupling I think this is why you can get signal through the amp with the volume all the way down. Without the tone stack the signal from the first tube would be 180 degrees out of phase with the second but with it? So I am not sure that this may not be a problem?

 

[Songman68]

Re

Step by step modifications done to Crate V5 amp

1) Remove the amplifier from cabinet. First remove speaker jack then the six screws holding back cover of amplifier then with amplifier upside down remove four screws two on each side of cabinet that hold amplifier chassis in cabinet. (Note: you may want to enlarge the holes slightly in sides of cabinet and on back cover to make reinstallation and future removal easier)

2) If you haven’t already done so (you do have this unplugged right?) get out a dc voltmeter the highest voltage in the amplifier is approx. 320 volts DC measure from chassis to R25 should be 0 to 15 volts if so you are safe. This amplifier has a bleeder resistor R25 so supplies should drop after power down.


3) Remove preamp (12AX7) and power tube (EL84 6BQ5) put them some where you wont break them.

4) Next remove knobs on front panel may require flat blade screwdriver and a thin piece of wood to work against to prevent scratching front panel. The pots are D shaft type walk then off gently. Then remove nuts holding pots and ¼ inch jack on front panel.


5) Now make note of all the connections to the PC board because you are going to remove them. If you have a digital camera now is a good time to use it. Also if you print out the schematic note wire color and location on it. The faston connectors J4 J5 J1 and J2 may be hard to remove don’t break them of pcb by working them back and fourth.

6) Next remove the six screws that hold PCB into chassis and take it out. (hope you are putting parts someplace you wont loose them)

PC Board modifications
(See attached pictures of mods and changes to schematic also pic DSCF0947.jpg in previous post is the pre emphasis mod the tone mod is incorrect in that post tie unused side of pot in tone control to wiper not ground or unconnected up to you)

1) You are going to be removing approx 10 parts. You will need solder, solder wick, soldering iron, needle nose pliers and desoldering tool. In manufacture the compnet leads have been bent so desolder a componet lead then straighten it before removing or you may damage PCB plated feedthrough.

2) This modification will use the two Op Amp stages for an input buffer and small gain to over come loss due to pre emphasis circuit that will be added. Desolder and remove volume pot from PCB. Next remove R13 this makes IC1-A a unity gain buffer. Remove C26, C24 and C27 replace C24 with a 47nF 50-100 volt poly cap.C24 sets the low frequency limit at input to the amplifier with 47nF this is approx 125 HZ 3db down (for more bass use 100nF). In the place of C27 install a 33k ¼ watt resistor this equals a gain of 1.22 for IC1-B (want to keep gain low so first stage tube will clip before the Op Amp with overdrive).


3) Pre emphasis circuit For the tone control to be able to cut, boost and be set for flat response the high frequencies need to be boosted first. So C25 needs to be removed if you want replaced or use it again for this mod. Install one lead of a 100nF 50-100 volt cap into plated hole (via) that is connected to pin 7 of IC1 in the second hole of C25 insert lead of 33k ohm ¼ watt resistor with a 10nf capacitor accost it then connect the other lead of the 33k resistor to the second lead of the 100nF capacitor. If you want full treble brighter you can lower value of R2

4) Tone stack and volume control Here we will put volume control after tone stack and first tube stage so you will be able to overdrive input tube and still control volume. Remove C5 C6 C28 and C4 Change C6 to a 22nF 50-100volt poly capacitor. (Here a lower value cap will raise where the tone control will cut the highs higher value will lower it.) Now the volume control remove R15 and R27 using the volume pot removed from the board you will need to bend the leads 90 degrees towards the back of the pot to attach 1 ½ inch wires to the three terminals of the pot. Also need to insulate PCB were pot was attached to prevent shorting (three layers of electrical tape works) looking at the front of the pot as use to be connected to the PCB the right side terminal connects to the plated hole C28 that connects to R29 the center terminal (wiper) connects to the plated hole of R27 that connects to R7 and control grid (pin 2) of the 12AX7. The last terminal goes to the plated hole of R27 that goes to ground. (note be careful the leads soldered to pot do not short to metal back shell of pot no sound out of amp)

5) Oscillating power tube in testing the amplifier found output did not clip symmetrically and oscillated when clipped. Based on the tube data sheets the cathode biasing is to high in value. Change R17 to 270-300 ohms 2-5 watt or parallel the 330 5 watt with another 1.5k ohm 2 watt (270 ohms)


6) That’s it now put it back together and be careful around high voltage always follow one hand rule working on tube amplifiers. The one hand rule is make sure other hand and body are not tied to ground other hand does all the probing. So tie meter or scope ground to the chassis ground then do all your measurements.

Other tweaks
From the way this amplifier is set up it may have had gain, tone and volume controls?
And they took the volume control out instead of the gain control so if you add a third 250k pot across TL01-B same as in the stock schematic this will be a gain control you should add a 27k resistor in series to set minimum gain at unity (1).

Lowest frequency amplified the stock value of C24 is to low 3db down at approx 270Hz the value in the mod is 47nF for 125Hz if you want you can go bigger 100nF but this may increase that annoying 60 Hz hum we have all come to love.

On the pre emphasis mod you can change the value of R2 to increase or decrease the brightness of the maximum tone. Higher value for R2 (stock 100k) will reduce the maximum lower value increase it. If you lower the value you may want to increase the gain in the Op Amp stage to offset the loss.

Tone control pot from my results the tone is flat at the 9 o’clock position so if you use a 100k (125K) pot for the tone control this will move it to the 12 o’clock position.
Changing where the tone control operates you can reduce the value of C6 to raise or lower where the tone control operates but you should change the value of the cap in the pre emphasis circuit also (10nF across 33k ohm).

It may be my lack of skills but I tried all of this and for some reason it didn't work right. I had really low volume and really bad distortion with the amp wide open.